What Is a Velvet Fork?
A velvet fork is a mechanism that does not require "a change in the rules at the consensus level," so it does not need approval by all network members. In addition, such forks allow for "consistent deployment" of the software and do not harm miners who did not support the update, and in this sense, they have similar features with soft forks.
Velvet forks were described for the first time in the research work of Non-Interactive-Proofs-of-Proof-of-Work (NIPoPoWs) on how to reach a consensus and published on December 4, 2017, by Aggelos Kiayias, Andrew Miller, and Dionysis Zindros.